1. Field of the Invention
The present general inventive concept relates to a method and apparatus to extract an important frequency component of an audio signal and a method and apparatus to encode and/or decode an audio signal by using the same, and more particularly to a method and apparatus to provide a high quality audio signal by effectively compressing data to a low bit rate. The method of encoding and decoding data may be utilized in a telecommunication apparatus or a signal processing apparatus, such as a mobile phone, a computer, a portable device, a display device, a recording and/or reproducing device, etc., which compresses an audio signal at a high compression rate and decompresses an audio signal having a high quality sound.
2. Description of the Related Art
An MPEG audio is a standard format of ISO/IEC for high quality and efficient stereo encoding. Subband encoding based on 32 bands (band division encoding) and modified discrete cosine transformation (MDCT) are used in audio signal compression, and highly efficient compression can be realized using psychoacoustic characteristics. MPEG audio can be achieved having more high quality sound by using the above technology.
In the MPEG audio, in order to efficiently compress an audio signal, a perceptual encoding compression method is used to reduce the amount of encoding by omitting low sensitive detailed data and applying human sensing characteristics for perceiving signals. Moreover, the perceptual encoding compression method using the psychoacoustic characteristics in the MPEG audio uses minimum audible limits and mask characteristics in a quiet environment. The minimum audible limit in a quiet environment is a minimal level of human auditory perception, is related to a noise limitation of the human auditory perception in a quiet environment, and is changed according to sound frequencies. At a certain frequency, a sound larger than the minimum audible limit can be heard, but a sound smaller than the minimum audible limit cannot be heard. Moreover, a sensing limitation of a specific sound is changed according to another sound. This is called a masking effect. A frequency width causing the masking effect is called a critical band. In order to effectively utilize psychoacoustic characteristics such as the critical band, it is important to divide audio signals into frequency components. For this, a band is divided into 32 bands for subband encoding. Additionally, at this point, the MPEG audio utilizes a filter bank to reduce aliasing noise of the 32 bands.
The MPEG audio includes bit allocation using the filter bank and a psychological sound mode, and quantization. A coefficient generated by a MDCT result is compressed by using a psychoacoustic model-2 by allocating an optimized quantization bit. Since the psychoacoustic model-2 for allocating an optimized bit calculates the masking effect by using a fast Fourier transform (FFT) and a spreading function, a high level of complexity is required.
When compressing an audio signal to a low bit rate (below 32 kbps), the number of bits allocated for each signal is insufficient for quantizing and encoding all frequency components of the audio signal. Accordingly, perceptually important frequency components need to be effectively extracted and encoded.
In a conventional method of extracting the perceptually important frequency component, and compressing and encoding the extracted component, an important frequency component and a noise component are separated and encoded by considering a psychoacoustic aspect. Additionally, a frequency component is reduced so as to apply a psychoacoustic model for the reduced frequency component by considering an output energy according to a frequency band of an audio signal.
However, when a conventional encoding method is used, a relatively large number of bits are required to specify an important frequency component. Moreover, since an important valley portion has a low signal-to-masking ratio (SMR) and energy in a voice signal, it is not selected as an important frequency component. Therefore, there is a limitation in providing a perceptually excellent audio signal.